The invention concerns the use of a magnetic bearing to position a body rotating relative to a stator body and is more particularly concerned with the mechanical retention that is required in practice before the magnetic bearing is activated and/or to protect the magnetic bearing from vibration or excessive amplitude likely to damage it. The invention has preferable, but not exclusive, applications in a space environment (intense vacuum, hostile environment, especially thermally, low or even negligible gravity).
It is not possible to design a magnetic bearing which can be passive (using only permanent magnets) both axially and in at least two transverse directions. This is well known. It is always necessary to provide, on one axis at least, active control using an electromagnetic actuator, in practice embodying a coil which is energized to generate in air gaps correcting fluxes which induce a correcting force conditioned by measurement signals supplied by position or speed sensors and set point values for position along the active control axis.
In the absence of any applied current, the rotating body departs from its unstable equilibrium position parallel to the axis on which control is to be applied. For this reason it is beneficial, in particular to avoid any excessive excursion, to provide mechanical abutments against which the rotating body bears in the absence of current in the control loop. To this end it is beneficial to include in the control logic some logic specifically dedicated to the phase in which the rotating body is lifted off the abutments when the control loop is activated.
There are two major categories of magnetic bearings, namely active axial control and passive radial control bearings and passive axial control and active radial control bearings. In the former case, where the rotating body is in unstable equilibrium parallel to the rotation axis, it is known to provide axial thrust bearings which can be mobile so that it is possible to key the rotating body axially. In the latter case, in which the rotating body is in unstable equilibrium transverse to the rotation axis, either the rotating body is allowed to depart from its usual set point radial position to bear radially against the abutments or, as previously, the rotating body is retained by axial keying.
The types of mechanical retention referred to above cannot always be used, either for reason of limited axial overall dimensions or because of their inability to retain the rotating body firmly in the presence of high levels of vibration, of the type generated in a satellite during launch, for example, or during firing or orbit correction thrusters.